Murfreesboro is a city in Pike County, Arkansas, United States. The population was 1,764 at the 2000 census. The city is the county seat of Pike County. Crater of Diamonds State Park is located just to the south. The crater is an eroded lamproite pipe where diamonds were discovered in 1906. 100,000 were found by 1933, and visitors to the park may still take away whatever they can find.
The guard cells surround each stoma in a plant cell. They regulate  the opening and closing of stomata to facilitate gas exchange and  control transpiration in plants.
When potassium ions (K+) are pumped into the guard cells the water potential in the cells decreases and water passes out of adjacent cells into the guard cells by osmosis. The guard cells become turgid (swollen with water). This opens the stomata and allows water loss (and gas exchange). Find out more by reading In and out of cells. Gas exchange. In the dark the plant's potassium pumps stop and potassium ions diffuse back out of the guard cells, they lose turgidity and the stomata close.